Recently, attention has been focused on liquid lenses as optical members for the autofocus function of cell phones with built-in cameras. As described in Nikkei Electronics, Oct. 24, 2005, pp. 129-134, “Non-mechanical member Amazing liquid lens near mass production stage”, a liquid lens has the following constitution: in a lens holder, two types of liquids, that is, an electroconductive aqueous solution and a nonconductive oil, are sealed; corresponding to the magnitude of the voltage applied on the aqueous solution, the shape of the interface between the aqueous solution and the oil is adjusted to realize a desired refractivity. By using a liquid lens as an image pickup lens, space for mechanically moving the lens position along the optical axis is not required, and the use of a motor or another actuator or movable member is not required, so the size and cost of the autofocus mechanism can be reduced.
FIG. 4 is a diagram illustrating the basic constitution of a liquid lens and the circuit constitution of a liquid lens driver in the prior art. For this liquid lens 100, lens holder 102 comprises a cylindrical or side wall portion of a three-layer structure consisting of upper metal electrode 104, insulator 106, and lower metal electrode 108, each in a ring shape, as well as upper transparent plate (window) 110 and lower transparent plate (window) 112. In lens holder 102, electroconductive aqueous solution 114 is sealed in the upper half, and insulating oil 116 is sealed in the lower half. The inner wall surface of said lower metal electrode 108 is tapered downward toward the center. Said insulator 106 covers the entirety of the inner wall surface of lower metal electrode 108, and it electrically insulates not only lower metal electrode 108 from upper metal electrode 104, but also from aqueous solution 114 and oil 116.
For liquid lens 100 with said constitution, when no voltage is applied between upper metal electrode 104 and lower metal electrode 108, the interface between aqueous solution 114 and oil 116 is almost flat. Then, when driving voltage VL (Out-P, Out-M) is applied from liquid lens driver 120 between two electrodes 104, 108, aqueous solution 114 is pulled toward lower metal electrode 108, so that oil 116 near the inner wall surface (slope) of lower metal electrode 108 is pressed out, and the pressed-out oil 116 is pulled toward the center. As a result, the interface between the two portions becomes curved, so the refractivity of liquid lens 100 for the light transmitted through it changes. By changing the magnitude of driving voltage VL (Out-P, Out-M), it is possible to control the curve shape or curvature of the interface, and it is possible to change the refractivity or focal distance.
Said liquid lens driver 120 has DC-DC converter 122, which converts prescribed DC voltage VDD input from a DC power source (not shown in the figure) to desired DC voltage Vs, oscillator 124, which oscillates and outputs a square-wave frequency signal or pulse signal CK, digital/analog converter (DAC) 126 for amplitude modulation, and control logic circuit 128 that controls said parts 122, 124, 126. Said DC-DC converter 122 is of the chopper system booster type having a switching element (not shown in the figure). During the ON period of the switching element, energy is accumulated in inductor 130, and, during the OFF period of the switching element, the energy is released from inductor 130 via diode 132 to the side of output capacitor 134.
Two resistors 136, 138 that are set parallel to output capacitor 134 to form a resistor voltage dividing circuit are connected in series. The connecting point or node N between the two resistors is connected to the input terminal of a feedback controller (not shown in the figure) in DC-DC converter 122, and, at the same time, it is connected to the output terminal of DAC 126. DAC 126 converts digital control signal VF input from control logic circuit 128 to analog control voltage Vf that is sent to node N. The potential at node N is changed by means of output voltage Vf of DAC 126, so converter output voltage Vs can be adjusted within a prescribed range (e.g., 10-60 V).
The two terminals of capacitor 134 are connected to the power source voltage input terminals of full bridge type output buffer circuit 140. Pulse signal CK at a prescribed frequency and having a prescribed waveform (prescribed ON/OFF time ratio) is supplied from oscillator 124 to the signal input terminal of output buffer circuit 140. Said output buffer circuit 140 divides converter output voltage Vs, input as the power source voltage, into bipolar, that is, positive/negative, output voltages Out-P, Out-M for output, and, corresponding to the H/L level of pulse signal CK, it turns ON/OFF said two output voltages Out-P, Out-M. In this way, from liquid lens driver 120, pulse amplitude modulation (PAM) output voltages Out-P, Out-M are applied as driving voltage VL on electrodes 104, 108 of liquid lens 100.
With said liquid lens driver 120, the amplitude of converter output voltage Vs and thus the amplitude of driving voltage VL (Out-P, Out-M) are adjusted by means of output voltage Vf of DAC 126, so that the refractivity of liquid lens 100 can be changed.
As explained above, in the liquid lens driving system in the prior art, liquid lens driver 120 with the constitution shown in FIG. 4 is used. By changing output voltage Vf of DAC 126 under control of control logic circuit 128, the amplitude of driving voltage VL (Out-P, Out-M) is changed as shown in FIG. 5, and the refractivity of liquid lens 100 can be changed.
However, for the liquid lens driving system, improvement still should be made on power consumption and response speed. That is, when said driving voltage VL (Out-P, Out-M) is changed from, e.g., 60 V to 30 V, output voltage Vf of DAC 126 falls to a prescribed value, capacitor discharge current is retrieved from the positive side terminal of capacitor 134 via resistor 136 to DAC 126, and converter output voltage Vs falls. In this case, if the resistance of resistor 136 is high, the discharge is slower, and the response speed, that is, the focusing speed, decreases. On the contrary, if the resistance of resistor 136 is selected to be smaller, the response speed is improved, while the power consumption rises. That is, in this case, while a rated voltage (60 V) is output to DC-DC converter 122, electric power is consumed by the resistors 136, 138 of voltage dividing resistor circuits, and the output falls to 30 V. Consequently, the power feeding efficiency falls. Also, even if DC-DC converter 122 can perform a high-speed feedback operation, setting the feedback constant is difficult, and the control of change in the output voltage (transient response) tends to be unstable.
In addition, because the output voltage of liquid lens driver 120, that is, driving voltage VL (Out-P, Out-M), depends on output buffer circuit 140, the desired dynamic range cannot be obtained. Also, because the voltage amplitude can be adjusted in analog format, it is likely to be influenced by the temperature characteristics. This is also undesired.
A general object of the present invention is to solve the aforementioned problems of the prior art by providing a liquid lens driving method, liquid lens driver, as well as an image pickup method and image pickup device using a liquid lens, characterized by the fact that it can realize higher speed, higher precision, higher efficiency and higher stability in driving of the liquid lens and to improve the autofocus function in an image pickup method and image pickup device using a liquid lens.